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Invasive Soviet water flea cuts US lake’s visibility by a metre

It’s an expensive invasion. The spiny water flea (Bythotrephes longimanus) has cut visibility in Lake Mendota in Wisconsin by 1 metre since it was first spotted there in 2009.

It did so by feasting on native plankton, which keep water clearer by eating algae. The economic costs of murkier water where residents enjoy boating, fishing and swimming has now been estimated at $140 million — though not everyone is convinced by such a high figure.

“Previous attempts to put a price tag on invasive species impacts haven’t come close to the true cost,” says study author Jake Walsh of the University of Wisconsin, Madison, who thinks the price tag justifies spending more money on eradicating the flea.

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The flea came to the lake from the Great Lakes, after having arrived there in the 1980s in the ballast water of ships, probably from the Soviet Union.

In a previous questionnaire, residents of the luxury homes along the lake shore said they would be willing to pay $640 per household for an extra metre of clarity in their lake. Walsh and his colleagues used this as a proxy for the economic cost of the invasion.

Fertile thinking

The team says the water could be made clearer by stripping phosphorus from farm fertilizer run-off into the lake. This could work because phosphorus boosts the growth of algae, which are making the water murky in the absence of large number of plankton there to eat them.

There is no known method of getting rid of the billions of fleas “short of poisoning the entire water body”, says the team.

And it may not be necessary. Nature could be about to rectify the problem – thanks to the arrival of another alien species that hitched a ride from the Soviet Union in the 1980s. Last year, the same university biology class that first spotted the spiny water flea in the lake came upon zebra mussels (Dreissena polymorpha) there.

Although the mussels can block water intakes and other infrastructure, they famously cleaned the murky waters of the heavily polluted Lake Erie after arriving there. “They likely will improve water clarity dramatically in Lake Mendota,” says Mark Davis of Macalester College in Minnesota.

But he is sceptical of the estimated cost. “I find it difficult to believe that Wisconsin citizens would be willing to pay $140 million to get that 1 metre of clarity back,” says Davis.